Is there any proof/evidence that Kaguya was a clan leader of otsutsuki?
We know Otsutsuki value power above all else. I don't think Otsutsuki will be the type that accept weak leader. Now Kaguya feared Momoshinki and Kinshiki. If Kaguya feared them, it stands to reason that her power, even after eating the fruit, was not enough for hear to feel safe. Therefore, her previous self, the one who didn't eat naruto's world chakra fruit, should be even weaker. How can she be a clan leader then?
If I recall correctly, Kaguya is from the main family line of the Otsutsuki clan. I don't think she was ever referred to as the leader of the Otsutsuki clan. The closest thing to a leader she could be, is the leader of the branch family of the Otsutsuki on earth.
There was no other Otsutsuki's on the planet. No other clan members, Kaguya becomes the matriarch of the Otsutsuki on earth and therefore clan leader. She doesn't have to be the leader of everyone with her name to be a leader of the clan.
She was a leader of the part of the clan that came to Earth to harvest the God Tree, according to the 4-th databook. Hamura inherited her position after her sealing and led these clan members to settle down on the Moon.
According to 4th databook, as stated above, she didn't arrive alone on Earth, but there were other Otsutsuki, most likely loyal to her, as well.
She is either ex-leader of all Otsutsuki or she was leader of the Otsutsuki on Earth, which it was, is not specified, same for her princesshoodl, if she was one before arriving on Earth, or got the title on Earth is also unknown.